This invention relates to inertial instruments, and more particularly to gyro configurations for inertial instruments.
The conventional wheel spin motor for both precision navigation grade and tactical grade gyros is a multi-phase hysteresis synchronous motor. This motor form includes wheel, or rotor, having a ring of high hysteresis magnetic steel which is torqued from a standstill to synchronous speed by interacting with a two- or three- phase lap- or wave-wound laminated electric motor stator. The ring is driven by exciting the stator windings from a fixed frequency supply reference.
The resultant motor is self-starting as the result of the hysteresis torques in the steel rotor ring. At synchronization, the rotor and stator magnetic fields are rotating at the same speed, although the rotor magnetic ring is no longer being dragged around its hysteresis curve, but rather maintains synchronous speed by virtue of the magnetic poles formed in the ring. New poles are formed each time the wheel is started. However, the ring is generally so magnetically soft that precise form of the rotor magnetic poles is a complicated function of the magnetic history of the stator flux and rotor position. In typical gyro applications, the magnetic history contributes substantially to gyro drift. In order to alleviate such problems, it is often necessary to employ complex control voltage amplitude sequences to the stator for systematically randomizing the rotor magnitization.
Although relatively simple in construction, the prior art hysteresis motors are relatively low in electrical efficiency (typically on the order of 60%). In addition to the efficiency problem, hysteresis motors are characterized by the following additional problems: Powerful attractive forces between stator and rotor require strict control air gap concentricity; multilaminated construction is needed to keep electrical losses as low as possible; and small air gaps are required to insure maximum efficiency (with resultant extreme tolerance requirements on the component parts).
Other prior art approaches utilize induction motors for driving gyro wheels. However, these systems are relatively inefficient particularly in view of eddy current losses in laminations and in addition, the eddy current losses provide a relatively high temperature environment which typically result in volatile material boiling off the windings which condenses on bearing surfaces. In addition, induction motors are characterized by relatively high slippage which results in the wheel not being strictly synchronous to the power supply. These induction motor configurations are characterized by relatively high power consumption.
In more recent developments, permanent magnet gyro wheel spin motors have been developed wherein a hard permanent magnetic rotor is used in a radial gap motor configuration. In this configuration, the primary change from conventional hysteresis motors is the substitution of a smooth ring of hard magnetic material (such as Alnico) for the hysteresis rotor. By way of example, the hysteresis ring of a GI-G6 gyro (Manufactured by Northrop Corporation, Norwood, Massachusetts) wheel motor may be replaced with an Alnico permanent magnet ring, together with a multiphased stator winding supported by a non-magnetic material. A further example of such a motor is described by D. E. Fulton in Report R-980, The Charles Stark Draper Laboratory, Inc., June, 1976, Cambridge, Massachusetts. In this configuration, the permanent magnet rotor is controlled to rotate at or very near the synchronous speed, for example, by an auxiliary induction motor run at a fixed-frequency. A closed loop network controls the amplitude of a stator field by continuously sensing the angle of the back emf (representative of the permanent magnet rotor position) to continuously control the angle of the state of magnetic field, and thereby commutate the motor.
Since the closed loop commutation circuit slaves the frequency angle of the stator current to the rotor position, full torque can be applied to the rotor at any frequency that provides enough back emf voltage to drive the back emf detection circuit. In this closed loop configuration, the torque angle can be operated at 90.degree., i.e. the difference between the stator and the rotor of magnetic field angles.
While this latter approach is relatively efficient (e.g. on the order of 90%), and avoids configurations having strong magnetic forces between stator and rotor, the radial gap permanent magnet motor of the prior art is a relatively complex physical configuration with relatively high difficulty of access to the component parts. Furthermore, the known radial gap permanent magnet motors are not well suited for certain applications such as wheel spin motors for free rotor (i.e. two degree of freedom) gyros.
It is an object of the present invention to provide a relatively compact permanent magnet motor.
Another object is to provide an axial gap, permanent magnet motor.
Yet another object is to provide a free rotor gyro having an axial gap permanent magnet wheel spin motor.
A further object is to provide a permanent magnet motor having a relatively high ratio of moment of inertia about its spin axis relative that about its input axes.